1. Field of the Invention
The present invention relates to radio antenna beam steering, and particularly to controlling the phases of radio frequency signals applied to corresponding antennas or antenna elements for beam steering.
2. Discussion of the Known Art
As illustrated in FIG. 1, when used as local oscillators (LOs) in a communications radio 10-1, RF synthesizers 12, 14 typically produce output signals LO1, LO2 the phases of which are locked to that of a common reference signal 16 supplied in the radio 10-1. If the transmitters in multiple transceivers or radios 10-1 to 10-N are used to excite N antennas or antenna elements 20-1 to 20-N of an antenna array 22, then the phase of each RF output signal 24-1 to 24-N applied to the antennas by the radios should be locked to a common or master reference signal so that a main beam of the array can be formed and steered accurately in a desired direction. See, e.g., R. C. Johnson, et al., Antenna Engineering Handbook, McGraw-Hill (1984), at pages 20-1 to 20-5. Due to phase shifts that commonly occur in one or more mixer stages 26, 28 of each radio transmitter during frequency conversions, and in subsequent RF circuits 30 leading to the antenna, the phase of an RF output signal 24-n applied at antenna 20-n (n=1 to N) is no longer locked to that of the reference signal 16 applied to the RF synthesizers 12, 14 in the radio 10-n. 
A need therefore exists for an arrangement and technique by which the phases of each of the RF output signals 24-1 to 24-N applied by the radios to the antennas 20-1 to 20-N can be accurately set to a desired value relative to a common phase reference.